This invention relates to internal combustion engines of the reciprocating piston type, and concerns the formation of the inlet ports in the cylinder heads of such engines. The invention is particularly although not exclusively applicable to the inlet ports of both compression-ignition and spark-ignition engines of the liquid-fuel-injection type.
An object of the invention is to provide a method of producing inlet ports shaped to induce a high degree of swirl in the inlet gases entering the combustion chamber of the engine, to an acceptable degree of consistency. Engines which use swirl to assist combustion normally require a precise speed of swirl to achieve efficient combustion. If the degree of swirl in each cylinder of a multi-cylinder engine varies, then combustion in some of the cylinders will not be optimum. This is why consistency of swirl generation by inlet ports is important to achieve good fuel consumption and, in some cases, low exhaust emissions.
Two kinds of inlet ports which generate swirl are known, the first being the so-called helical inlet port, defined as being an inlet port in which the inlet gases are led through an inlet duct to enter a substantially annular portion of the port (referred to as the bowl) above the valve seat and around the valve stem, the inlet duct intersecting the bowl asymmetrically (with respect to the axis of the inlet valve and the valve seat) so that the outer side of the inlet duct merges smoothly with the internal surface of the wall of the bowl. Thus the inlet gases are introduced in a generally circumferential direction into the bowl from the inlet duct whose longitudinal axis is radially-offset from the "vertical" axis of the valve stem and valve seat, so that the gases will travel along a helical path around the valve stem in the bowl of the port and thereby acquire angular momentum before passing over the valve seat into the combustion chamber. A frequent though not indispensable feature of the helical inlet port design is the provision of a barrier or fin which protrudes into the bowl near the inner side of the inlet duct and deflects gas passing round the bowl, so avoiding interference with gas leaving the end of the duct and entering the bowl.
The arrangement of a helical inlet port is thus contrasted with that of a so-called "directed" inlet port in which the axis of the straight, downwardly inclined inlet duct intersects, or almost intersects, the axis of the valve stem in an arrangement which is symmetrical in plan, so that the inlet gases are biassed to enter the combustion space in one direction, or in other words the inlet gases have a linear momentum which is converted to the necessary angular momentum within the combustion chamber.
It has been the practice to form a helical inlet port by casting, on account of its complex shape, as a part of the operation of casting the cylinder head, leaving the surfaces of the port as cast, but it has been found that the degree of swirl which such cast ports produce is very sensitive to minor inaccuracies in their shape, such as can occur during a production run through slight displacement of the casting core. Moreover even the roughness of the cast surface can affect the degree of swirl produced.